In our copending application Ser. No. 581,178, filed on even date herewith, we have disclosed a decoding unit, located inside a room, which responds to a predetermined entry code fed in from the outside and matching a reference code stored in a memory of that unit in order to release a door-locking mechanism. As described in that concurrently filed application, the entry code can be generated by a keyboard on a device of the type here considered. The term "keyboard", as used here, encompasses not only an array of individually depressible keys or pushbuttons (such as the 12-button panels commonly used in telephones of the "touch-tone" type) but also assemblies wherein a frame is spanned by a flexible foil or membrane bearing markings on its outer surface which designate certain areas as pressure faces whose depression will close a set of underlying contacts; see, for example, German laid-open application No. 29 50 680 published June 19, 1981. The areas so designated as pressure faces, together with their associated contacts, can therefore also be referred to as keys.
In order to generate a certain code consisting of a predetermined number of bits, the user must actuate these keys in a prescribed manner. With a 24-bit code as described in our copending application, for example, the keyboard has six rows of four bits each and the user depresses in each row the key or keys corresponding to bits of a given logical value (either "0" or "1") in a respective 4-bit word. The words so generated are stored in a register and, after selection is completed, are jointly read out to the decoder on a multiplicity of leads, 24 of them in the example referred to.
A problem with such a keyboard resides in the possibility that an unauthorized person, such as a potential intruder, might be able to determine with the aid of known fingerprint-revealing methods which keys of a keyboard had recently been operated. A widely used technique for the detection of fingerprints resides in spreading a powder of contrasting color over the keyboard. New fingerprints are also readily recognizable if the keyboard had been previously treated with a degreasing solvent for the removal of old prints.
Especially in situations where the keys used to generate a certain code are to be depressed only once and where the sequence of their operation is not critical, a person resorting to these techniques could readily reconstruct the code giving access to a locked room.
This problem has been recognized in the above-identified German application which proposes to solve it by replacing the fixed markings on the pressure faces of the keys by controllable indicators of the liquid-crystal or light-emitting-diode type forming randomly variable symbols. The user, guided by the alphanumerical or other characters appearing at that particular time, will then generate the requisite data combination by the depression of keys in a sequence or pattern different from that required for the same code at an earlier or later occasion.
While the known system is undoubtedly effective to prevent the successful duplication of a secret code by an unauthorized observer, its implementation is expensive; also, the generation of the code is time-consuming since the user must always search for the new loaction of any key to be actuated.